Walter Mitty is a fictional character in James Thurber's short story The Secret Life of Walter Mitty, published in 1941. Mitty is a meek, mild man with a vivid fantasy life: in a few dozen paragraphs he imagines himself a wartime pilot, an emergency-room surgeon, and a devil-may-care killer. The term now appears in dictionaries to describe a person who lives a fantasy life.

A film version of the story was released in 1947 starring Danny Kaye and directed by Norman McLeod .

The term can be used as an insult, as in two cases arising in British politics:

• In 1977, Andrew Roth entitled his biography of former British prime minister Harold Wilson Sir Harold Wilson: the Yorkshire Walter Mitty. Wilson successfully sued Roth for libel arising out of a section of the book referring to Wilson's wife.

• In 2003, Tom Kelly, a spokesman for British prime minister Tony Blair, publicly apologised for referring to the late David Kelly as "a Walter Mitty character" during a private discussion with a journalist.